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We describe a case of catheter-related bacteremia due to Staphylococcus pseudintermedius in a child with dog exposure. The organism was confirmed as S. pseudintermedius based on 16S rRNA gene sequence analysis and positive PCR-restriction fragment length polymorphism of the pta gene.
A 6-year-old boy with hemophilia B was admitted due to intermittent high fever which developed on two successive occasions after heparin flushing of the Port-A-Cath (Smiths Industries Medical Systems [SIMS] Deltec, Inc., St. Paul, MN) used to deliver regular recombinant factor IX therapy. In the recent 2 months, the patient had helped raise two Maltese dogs at home. Erythema and tenderness over the Port-A-Cath insertion site were noted during the week before this visit. Laboratory examinations revealed a white cell count of 6,140/μl (with 54.2% neutrophils) and a C-reactive protein level of 2.59 mg/dl (reference, <0.8 mg/dl).
Two sets of blood cultures drawn from a peripheral vein and the catheter all grew Staphylococcus aureus (isolate A) presumptively identified by positive coagulase reaction using the rapid latex agglutination test (Staphaurex Plus; Remel Laboratories, Lenexa, KS) and conventional methods. Antibiotic-lock therapy with vancomycin (25 mg in 5 ml normal saline) via the catheter and intravenous oxacillin (100 mg/kg/day) was administered. Fever subsided gradually. However, blood culture drawn from the catheter before (isolate B) and those drawn at the 48th (isolate C), 72nd (isolate D), and 120th (isolate E) hour after antibiotic-lock therapy still grew the same organisms. The catheter was removed with some pus surrounding the insertion site. Culture and Gram staining results of the pus were negative for bacteria. No intracardiac vegetation was found by transthoracic echocardiography. Oxacillin (100 mg/kg/day) was given for a total of 18 days, and the patient recovered uneventfully.
Because of the patient's dog exposure history and the clinical importance of the sample, further identification methods were performed. Five isolates (isolates A to E) were identified as Staphylococcus intermedius by the Phoenix automated system PMIC/ID-30 (sequence number 425800151413; confidence value, 99%) (Becton Dickinson Diagnostic Systems, Sparks, MD) and Vitek II (code number 050412075723231; probability of identification, 99%) (bioMérieux Inc., La Balme les Grottes, France). However, these two identification systems do not include Staphylococcus pseudintermedius and Staphylococcus delphini in their databases. These organisms were further confirmed to the species level by using 16S rRNA gene sequence analysis. The sequences obtained (1,075 bp) were compared with published sequences in the GenBank database by using the BLASTN algorithm (http://www.ncbi.nlm.nih.gov/blast). The closest match was obtained with S. pseudintermedius (accession number AJ780977.1; maximal identity, 99% [1,074/1,075 bp]). Further pta gene analysis was also performed following a previous description (1). The isolates exhibited positive PCR-restriction fragment length polymorphism of the 320-bp fragment in the pta gene, which was compatible with the identification of S. pseudintermedius (1). These isolates were resistant to polymyxin E (10-g colistin disk) by the disk diffusion method, which was also in accordance with the identification of S. pseudintermedius (6). The MICs of these isolates to penicillin, oxacillin, ciprofloxacin, and vancomycin had identical values of >0.25 μg/ml, ≤0.25 μg/ml, ≤0.25 μg/ml, and ≤1 μg/ml, respectively, as determined by the Phoenix automated system. Molecular typing of the five isolates by pulsed-field gel electrophoresis using SmaI (Sigma Chemical Co., St. Louis, MO)-digested DNA disclosed identical pulsotypes, indicating the persistence of bacteria in the catheter during antibiotic-lock therapy.
Biochemically, S. pseudintermedius can easily be misidentified as S. aureus (2-4). Isolates phenotypically closely related to S. intermedius include S. pseudintermedius and S. delphini (1, 2, 4). S. pseudintermedius is one of the most common pathogens isolated from skin and ear infections in dogs (3-5). Van Hoovels et al. (6) reported the first case of S. pseudintermedius human infection in 2006. This pathogen was isolated from an infected implantable cardioverter defibrillator from a patient without animal contact history (6). No other human infections caused by this organism have been previously reported. Both the present case and this previously reported case had infections related to an intravascular device which was removed prior to clinical and microbiological improvement (6). Although several quorum-sensing mechanisms have been reported for S. pseudintermedius, the capacity of the organism to form a biofilm remains unknown. The MIC interpretive criteria for susceptibility testing of this veterinary organism remain to be established. The S. pseudintermedius isolate from our patient exhibited a low MIC value to oxacillin; however, strains of this organism resistant to methicillin have been isolated from animals (7).
Because S. pseudintermedius may be confused with S. aureus or misidentified as S. intermedius by routinely used biochemical identification systems, clinical microbiology laboratories should include genotypic identification methods and several biochemical tests to differentiate S. aureus (S. pseudintermedius lacks pigment and clumping factor activity and exhibits a weak and delayed mannitol fermentation; reaction is positive for pyrrolidonyl arylamidase and o-nitrophenyl-β-d-galactopyranoside [ONPG; β-galactosidase]) and S. intermedius (S. pseudintermedius has negative arginine dihydrolase and acid production from β-gentiobiose and d-mannitol) from related species (1, 2, 5, 6).
In summary, we report a case of catheter-related bacteremia due to oxacillin-susceptible S. pseudintermedius refractory to antibiotic-lock therapy in a child with dog exposure. Accurate identification of the organism requires the combination of phenotypic and genotypic tests. This case expands the reported spectrum of persistent infection caused by S. pseudintermedius to include catheter-related bacteremia.
Published ahead of print on 17 February 2010.